I tend not to take any money with me to the beach. If it can't be avoided, then I take the smallest amount possible and tuck it into the pocket of my shorts (NOT in a wallet which is too obvious).
If I'm feeling particularly vulnerable, I have tucked it into a small waterproof bag (one that I would normally use for my camera to take photos while snorkelling).
I think they are also smaller waterproof plastic cases that you can get to carry around your neck to hold a room key and a small amount of cash. Haven't tried one though.
I would always leave my main wallet back in the room, locked up in my pack.
I think the best way is to buy waterproof bag for mobile phone in which you can put on
banknotes and coins.
in many cases these are the strings that you can get it hanging on your neck or hand or something.